László Zékány

Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions

Interaction between iron(II) and acetohydroxamic acid (Aha), alpha-alaninehydroxamic acid (alpha-Alaha), beta-alaninehydroxamic acid (beta-Alaha), hexanedioic acid bis(3-hydroxycarbamoyl-methyl)amide (Dha) or desferrioxamine B (DFB) under anaerobic conditions was studied by pH-metric and UV-Visible spectrophotometric methods. The stability constants of complexes formed with Aha, alpha-Alaha, beta-Alaha and Dha were calculated and turned out to be much lower than those of the corresponding iron(II) complexes. Stability constants of the iron(II)-hydroxamate complexes are compared with those of other divalent 3d-block metal ions and the Irving-Williams series of stabilities was found to be observed. Above pH 4, in the reactions between iron(II) and desferrioxamine B, the oxidation of the metal ion to iron(III) by the ligand was found. The overall reaction that resulted in the formation of the tris-hydroxamato complex [Fe(HDFB)]+ and monoamide derivative of DFB at pH 6 is: 2Fe2+ + 3H4DFB+ = 2[Fe(HDFB)]+ + H3DFB-monoamide+ + H2O + 4H+. Based on these results, the conclusion is that desferrioxamine B can uptake iron in iron(III) form under anaerobic conditions.

Slow dynamics of aluminium-citrate complexes studied by 1H- and 13C-NMR spectroscopy

Abstract Inter- and intra-molecular exchange reactions of the major Al(III)-citrate (Cit) complexes have been studied under equilibrium conditions in aqueous solution by 1H- and 13C-NMR using band shape analysis and magnetization transfer methods. Rate equations and activation parameters have been evaluated from concentration, pH and temperature dependent studies. The lability with respect to ligand exchange was: Al ( Cit ) 2 3− (k 298 =1.1±0.1 s −1 )> Al 3 ( H −1 Cit ) 3 ( OH ) 4 7− (Sy; k 298 =0.08±0.01 s −1 )≫ Al 3 ( H −1 Cit ) 3 ( OH ) 4− (As) . The variation in lability appears to be related to the different coordination modes of the citrate ligands in the complexes. The ligand exchange reactions show an Ia mechanism for Al(Cit)23− and Sy. These two complexes are fluxional (k 298 Sy =230 s −1 ) , the intra-molecular rearrangement of Sy goes through a bond rupture mechanism. As is neither fluxional nor labile for ligand exchange, kAs≤0.03 s−1 at 353 K.

Solution Structures, Stabilities, Kinetics, and Dynamics of DO3A and DO3A−Sulphonamide Complexes

The Gd(3+)-DO3A-arylsulphonamide (DO3A-SA) complex is a promising pH-sensitive MRI agent. The stability constants of the DO3A-SA and DO3A complexes formed with Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) ions are similar, whereas the logKLnL values of Ln(DO3A-SA) complexes are 2 orders of magnitude higher than those of the Ln(DO3A) complexes. The protonation constant (log KMHL) of the sulphonamide nitrogen in the Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) complexes is very similar to that of the free ligand, whereas the logKLnHL values of the Ln(DO3A-SA) complexes are lower by about 4 logK units, indicating a strong interaction between the Ln(3+) ions and the sulphonamide N atom. The Ln(HDO3A-SA) complexes are formed via triprotonated *Ln(H3DO3A-SA) intermediates which rearrange to the final complex in an OH(-)-assisted deprotonation process. The transmetalation reaction of Gd(HDO3A-SA) with Cu(2+) is very slow (t1/2 = 5.6 × 10(3) h at pH = 7.4), and it mainly occurs through proton-assisted dissociation of the complex. The (1)H and (13)C NMR spectra of the La-, Eu-, Y-, and Lu(DO3A-SA) complexes have been assigned using 2D correlation spectroscopy (COSY, EXSY, HSQC). Two sets of signals are observed for Eu-, Y-, and Lu(DO3A-SA), showing two coordination isomers in solution, that is, square antiprismatic (SAP) and twisted square antiprismatic (TSAP) geometries with ratios of 86-14, 93-7, and 94-6%, respectively. Line shape analysis of the (13)C NMR spectra of La-, Y- , and Lu(DO3A-SA) gives higher rates and lower activation entropy values compared to Ln(DOTA) for the arm rotation, which indicates that the Ln(DO3A-SA) complexes are less rigid due to the larger flexibility of the ethylene group in the sulphonamide pendant arm. The fast isomerization and the lower activation parameters of Ln(DO3A-SA) have been confirmed by theoretical calculations in vacuo and by using the polarizable continuum model. The solid state X-ray structure of Cu(H2DO3A-SA) shows distorted octahedral coordination. The coordination sites of Cu(2+) are occupied by two ring N- and two carboxylate O-atoms in equatorial position. The other two ring N-atoms complete the coordination sphere in axial positions. The solid state structure also indicates that a carboxylate O atom and the sulphonamide nitrogen are protonated and noncoordinated.

A NEW CLASS OF OLIGONUCLEAR PLATINUM-THALLIUM COMPOUNDS WITH A DIRECT METAL-METAL BOND. 3. UNUSUAL EQUILIBRIA IN AQUEOUS SOLUTION

A new series of four binuclear platinum-thallium cyano compoundscontaining a direct and unsupported by ligands metal2metalbond has been prepared in aqueous solution. Thesecompounds are represented by the formula[(NC)5Pt2Tl(CN)n21](n21)2 (n = 124 for compounds I, II, IIIIV, respectively) and [(NC)5Pt2Tl2Pt(CN)5]32 (for compoundV). The oligonuclear complexes are synthesised accordingto the reaction mPt(CN)422 + Tl3+ + nCN2 v [PtmTl-(CN)4m+n]322m2n. Thus, there occurs a change of the coordinationnumber of the Pt center from four (square planar) tosix (octahedral). Consequently, the formation of binuclearplatinum-thallium cyano compounds involves at least twosteps: (i) formation of metal2metal bond and (ii) formation of(NC)5Pt2 unit by a cyanide transfer process. 2 The complexes exist in an equilibrium, which also includes the parentcomplexes Pt(CN)422 and Tl(CN)n32n (n = 024), and can becontrolled by varying the cyanide concentration and/or pHof the solution. The stability constants of the compounds âN =[PtmTl(CN)4m+n322m2n] / {[Pt(CN)422]m· [Tl3+] ·[CN2]n} havebeen determined by means of multinuclear NMR (195Pt,205Tl): logâN = 19.9±0.4, 30.7±0.3, 38.6±0.3, and 44.8±0.2 forI, II, III, and IV (m = 1, n = 124), and 32.1±0.3 for V (m = 2,n = 2), respectively, (in 1 M NaClO4 as ionic medium, at 25oC). To our knowledge, the present work constitutes the firstdetailed equilibrium study of metal2metal bonded compounds;it indicates that also other cluster formation reactionsdescribed in the literature may represent real equilibria.

Equilibrium study of the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate, 3-mercaptopropionate and 2-mercaptobenzoate

Abstract Equilibrium studies were carried out by pH-potentiometry on the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate (MerAc2−), 3-mercaptopropionate (MerPr2−) and 2-mercaptobenzoate (MerBe2−). It was found that the complex-forming properties of the Al3+ ion towards these mercaptocarboxylic acid ligands differ from those of Ga3+ and Al3+. Under the conditions of the study, Al3+ forms only hydroxo complexes, while Ga3+ and In3+ form relatively stable complexes involving the simultaneous coordination of the carboxylate and the deprotonated mercapto group. In all cases the equilibrium systems can be described without the assumption of polynuclear complexes. The complexes Ga(MerAc)2 and Ga(MerBe)2 show marked stability; this was interpreted in terms of back-coordination and of interaction between the d10 electrons of the Ga3+ ion and the empty d orbitals of the S donor atom. Complexes of composition MLi are not formed in the Ga3+-MerPr2− system; this points to the importan roles of the number of atoms in the chelate ring and the higher stability of the Ga(III)-hydroxo complexes.

The Role of Equilibrium and Kinetic Properties in the Dissociation of Gd[DTPA‐bis(methylamide)] (Omniscan) at near to Physiological Conditions

[Gd(DTPA-BMA)] is the principal constituent of Omniscan, a magnetic resonance imaging (MRI) contrast agent. In body fluids, endogenous ions (Zn(2+), Cu(2+), and Ca(2+)) may displace the Gd(3+). To assess the extent of displacement at equilibrium, the stability constants of DTPA-BMA(3-) complexes of Gd(3+), Ca(2+), Zn(2+), and Cu(2+) have been determined at 37 °C in 0.15 M NaCl. The order of these stability constants is as follows: GdL≈CuL>ZnL≫CaL. Applying a simplified blood plasma model, the extent of dissociation of Omniscan (0.35 mM [Gd(DTPA-BMA)]) was found to be 17% by the formation of Gd(PO4), [Zn(DTPA-BMA)](-) (2.4%), [Cu(DTPA-BMA)](-) (0.2%), and [Ca(DTPA-BMA)](-) (17.7%). By capillary electrophoresis, the formation of [Ca(DTPA-BMA)](-) has been detected in human serum spiked with [Gd(DTPA-BMA)] (2.0 mM) at pH 7.4. Transmetallation reactions between [Gd(DTPA-BMA)] and Cu(2+) at 37 °C in the presence of citrate, phosphate, and bicarbonate ions occur by dissociation of the complex assisted by the endogenous ligands. At physiological concentrations of citrate, phosphate, and bicarbonate ions, the half-life of dissociation of [Gd(DTPA-BMA)] was calculated to be 9.3 h at pH 7.4. Considering the rates of distribution and dissociation of [Gd(DTPA-BMA)] in the extracellular space of the body, an open two-compartment model has been developed, which allows prediction of the extent of dissociation of the Gd(III) complex in body fluids depending on the rate of elimination of the contrast agent.

Equilibrium and structure of thallium(III) -ethylenediamine complexes in pyridine solution and in solid

The formation of three [TI(en)(n)](3+) complexes (n = 1-3) in a pyridine solvent has been established by means of Tl-205 and H-1 NMR. Their stepwise stability constants based on concentrations, K-n ...

Equilibrium studies on the AlIII-, GaIII-, InIII- and TlIII-ethylenediaminetetraacetate-halide and -sulphide systems

Abstract The formation of mixed ligand complexes was studied in the Al(edta)−, Ga(edta)−, In(edta)− and Tl(edta)−-halide and -sulphide ion systems by potentiometry, using glass and anion-selective electrodes. The stability constants (log KMLF) of the mixed ligand complexes M(edta)F2− were found to be 4.8±0.2, 1.9±0.1 and 0.9±0.1 for Al3+, Ga3+ and In3+, respectively. Values of log KMLX = 2.3±0.2, 3.5±0.15 and 5.9±0.2 were measured for Tl(edta)X2−, X = Cl−, Br− and I−, respectively. Complexes M(edta)S3− are described. The values of log KMLS were calculated to be 10.3, 10.6 and 9.4 for Al3+, Ga3+ and In3+, respectively. Ionic-covalent interactions in the mixed complexes are discussed. The stability constant of Tl(edta)− was reinvestigated via spectrophotometric study of the competition reaction in the Tl3+-H+-(edta)4−-X− system.

Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, 13C and 19F NMR spectroscopy

The AlOx1–3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by 13C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx2− complex (k(298 K) = 5 s−1, ΔH# = 67 ± 5 kJ mol−1, ΔS# = −6 ± 6 J mol−1 K−1, the rate determining step could be the breaking of the Al–O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx33− complex and the free ligand (k30(298 K) = 6.6 · 10−5 s−1, ΔH# = 164 ± 17 kJ mol−1, ΔS# = 225 ± 51 J mol−1 K−1, D/Id mechanism) were determined by dynamic 1D and 2D 13C NMR measurements. Mixed complexes, AlFOx, AlFOx22–, AlF2Ox−, and AlF2Ox23–, with overall stability (logβ) of 11.53 ± 0.03, 15.67 ± 0.03, 15.74 ± 0.02, and 19.10 ± 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by 13C and19F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox2–-F− complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.

Comparative study of hydroxo-fluoro and hydroxo-sulphido mixed ligand complexes of aluminum(III) and gallium(III)

Abstract The mixed ligand complex formation had been studied in the aluminate-fluoride, gallate-fluoride, aluminate-sulphide and gallate-sulphide systems by spectrophotometric as well as potentiometric methods using glass and sulphide selective electrodes. The formation of the species Al(OH) 3 F − and Ga(OH) 2 S − has been demonstrated and the equilibrium constants have been determined. Similar interaction could not be detected in the aluminate-sulphide and gallate-fluoride systems. The differences in behaviour of the metal ions to the characteristically hard fluoride and soft sulphide ligands can be interpreted in that the Ga 3+ ion may form complexes with charged soft ligands too, its behaviour then differing from that of the typically hard Al 3+ ion.